1. Field of the Invention
The present invention relates to an abnormality detection apparatus of an injection molding machine.
2. Description of the Related Art
In a mold opening-closing operation or molded-product ejecting operation in an injection molding cycle for manufacturing molded products using an injection molding machine, a reference load of a motor for driving a moving part is first stored in association with the time or the position of the moving part. Then, the stored reference load and an actual load of the motor are successively compared in association with the time or the position of the moving part. Abnormality in the mold opening-closing operation or ejecting operation is detected based on whether or not a preset threshold is exceeded by a deviation between the stored reference load and an actual load, whereupon the operation of the injection molding machine is stopped at once. In this way, a mold and mechanical parts of the injection molding machine are prevented from being broken.
Abnormality detection techniques for such breakage prevention are disclosed in, for example, Japanese Patent Applications Laid-Open Nos. 2001-30326 and 2001-38775. In these techniques, a load for one past cycle or a load determined by calculating the moving average of loads for a plurality of past cycles during which normal mold opening-closing operations or normal ejecting operations are performed is set as a reference load. According to the techniques disclosed in these patent documents, however, an operator must set a threshold for abnormality detection, and an operation for setting the threshold may be a burden to the operator.
Further, control techniques for an injection molding machine are disclosed in Japanese Patent Applications Laid-Open Nos. 2004-330529 and 2005-280015. In these techniques, a threshold is determined based on the average and variance of motor currents detected in the past. In calculating the average and variance of the motor currents detected in the past, according to the techniques disclosed in these patent documents, however, a large storage capacity is needed to store detected values for a predetermined shot count, and arithmetic processing of plenty of stored detected values requires a massive computation.